// Production steps of ECMA-262, Edition 5, 15.4.4.18
// Reference: http://es5.github.io/#x15.4.4.18
Array.prototype.forEach = function(callback, thisArg) {
  
  var T, k;

  if (this == null) {
    throw new TypeError(' this is null or not defined');
  }

  // 1. Let O be the result of calling toObject() passing the
  // |this| value as the argument.
  var O = Object(this);

  // 2. Let lenValue be the result of calling the Get() internal
  // method of O with the argument "length".
  // 3. Let len be toUint32(lenValue).
  var len = O.length >>> 0;

  // 4. If isCallable(callback) is false, throw a TypeError exception.
  // See: http://es5.github.com/#x9.11
  if (typeof callback !== "function") {
    throw new TypeError(callback + ' is not a function');
  }

  // 5. If thisArg was supplied, let T be thisArg; else let
  // T be undefined.
  if (arguments.length > 1) {
    T = thisArg;
  }

  // console.log(T);

  // 6. Let k be 0
  k = 0;

  // 7. Repeat, while k < len
  while (k < len) {

    var kValue;

    // a. Let Pk be ToString(k).
    //    This is implicit for LHS operands of the in operator
    // b. Let kPresent be the result of calling the HasProperty
    //    internal method of O with argument Pk.
    //    This step can be combined with c
    // c. If kPresent is true, then
    if (k in O) {

      // i. Let kValue be the result of calling the Get internal
      // method of O with argument Pk.
      kValue = O[k];

      // ii. Call the Call internal method of callback with T as
      // the this value and argument list containing kValue, k, and O.
      // console.log(T);
      callback.call(T, kValue, k, O);
    }
    // d. Increase k by 1.
    k++;
  }
  // 8. return undefined
};

  const a = [ ]

  a[1]=2
  a[3]=4
  a.forEach(item=>console.log(item))